Recently, an active matrix display device (e.g., a liquid display device, a luminescent display device, and an electrophoretic display device), in which a switching element formed of a thin film field-effect transistor (FET) is provided in each of display image elements arranged in a matrix form, has been actively developed.
In these developments, attentions have been paid on techniques for forming FET using an oxide semiconductor film, which has high carrier mobility, and less variation between elements, in a channel forming region of the FET, and applying such FET to an electron device, an optical device, etc. For example, proposed is FET using, as an oxide semiconductor film, zinc oxide (ZnO), In2O3, In—Ga—Zn—O, etc.
There are currently needs for display devices having large display areas. In a display device for displaying in a large area, a problem of signal delay due to resistance occurred from the wiring to a channel of the FET becomes significant. Therefore, in production of FET, materials having low resistivity are ideally used as materials of wiring, source electrode, and drain electrode. Examples of such materials include metal.
In the case where FET has a structure in which a source electrode and drain electrode both formed of metal having low resistivity are directly in contact with an oxide semiconductor film, contact resistance thereof increases. It has been considered that one of the factors for increasing the contact resistance is formation of Schottkey barrier junction on a contact surface of the oxide semiconductor film with the source electrode and drain electrode.
Therefore, it is desired to provide an electrode having low resistivity, and form an excellent electrical connection with an oxide semiconductor film.
In order to solve the aforementioned problems, disclosed is a method for providing a buffer layer for forming ohmic contact between a source electrode and drain electrode, and an oxide semiconductor layer, and using metal having low resistance for the electrodes (see PTL 1). In accordance with the disclosed method, FET having excellent properties can be produced. However, as this method requires a buffer layer, a number of the production processes of the FET increases.
Moreover, disclosed is FET using electroconductive oxide, such as thin-doped indium oxide (ITO), and zinc-doped indium oxide (IZO), for a source electrode and drain electrode, and using oxide semiconductor for an active layer (see PTLs 2 and 3). However, the resistivity of the electroconductive oxide is generally higher than that of metal. Therefore, the disclosed technique cannot achieve both low resistivity and excellent electrical connection with an oxide semiconductor film.
Accordingly, there are currently needs for an electroconductive thin film having low resistivity, and is capable of forming an excellent electric connection with an oxide semiconductor film without increasing stages of a production process.